Field of the Invention
This invention relates to a thermal conductivity sensing device, to a method for operation of the thermal conductivity sensing device and to uses of the thermal conductivity sensing device. The invention is of particular interest as a sensor in a gaseous environment, for sensing thermal conductivity of the gaseous environment which can provide information on the gas components. The invention has particular, but not exclusive, applicability in the fields of gas chromatography and/or medical analysis.
Related Art
Gas chromatography is a technique which is used to separate and detect the components of a mixture of gases. Gas chromatography is typically carried out using a separation column. The gas mixture to be separated into its constituent components is carried through the column using a carrier gas (‘mobile phase’). The column is provided with a stationary phase (e.g. a coating on an inner surface of the column). The stationary phase retards the different components of the gas mixture to different extents, in a conventionally-known manner. As a result, the different components are passed through the separation column at different rates, and elute from the column at different, characteristic times, known as retention times.
As the separated components elute from the column, a detector presented to the gas flow detects the components eluted from the column. Suitable detectors are known which detect the thermal conductivity of a gaseous environment. These are referred to in the art as thermal conductivity detectors (TCDs). These rely on the fact that different gas components have different thermal conductivities, and in particular, a different thermal conductivity from the mobile phase. As one component reaches the detector, the change in thermal conductivity of the gaseous environment registers as a peak. The change in thermal conductivity of the gaseous environment, along with the retention time, can then be used to identify the component.
One benefit of TCDs is that they do not rely on any kind of chemical reaction. They are able to detect not only the presence of the different components in the environment, but given the identity of the expected components, they can also provide information related to the concentration of the components.
The basic operating principle of a typical known TCD is to have a heated filament located to be in thermal contact with a gaseous analyte. A change in composition of the analyte typically changes the thermal conductivity of the analyte. Therefore the rate at which the heated filament loses heat to the analyte also changes, resulting in a change in temperature of the heated filament. This change in temperature is usually measured as a change in electrical resistance of the heated filament. A well-known example of a device which relies on thermal conductivity to measure low gas pressures is a Pirani gauge, which has a heated filament exposed to the gas. The lower the pressure, the lower the rate of heat loss to the surroundings. Therefore, by measuring the temperature of the filament, one can infer the gas pressure (i.e. the extent of the vacuum).
As mentioned above, TCDs can be used in gas chromatography, but can also be used in sensors for any mixture of gases, for example hydrogen and natural gas, or air and fuel in combustion systems.
Thermal conductivity gauge XEN-TCG3880 of Xensor Integration by (Distributieweg 28, 2645 EJ Delfgauw, Netherlands, www.xensor.nl) is a commercially-available sensor. It is a thin-film-thermopile TCD. The device has a heated membrane in contact with hot junctions of the thermopile. The cold junctions are connected to the thick rim of the chip.
WO 2011/044547 discloses a micro-TCD which is made up of a chamber with a suspended heater inside, with current contacts and voltage contacts. In one embodiment, there is a series of three heating elements located along the gas flow path. An analyte band flowing along the gas flow path is detected by the first element and subsequently by the second and third, the delay between the detection by each element providing a measure of the flow rate.
Kaanta et al. (2010) and Kaanta et al. (J. Micromech. Microeng 2011) discuss a sensor which utilizes thermal conductivity measurements to infer the flow rate of a sample gas within a detector. This device consists of several detector elements, which are all heated and measured simultaneously. This allows for direct measurement of the sample peak as it progresses through the microchannel of the detector. This works by maintaining the heated filament at a constant temperature, and monitoring the power required to maintain the temperature. The lower the power, the lower the thermal conductivity of the sample. The feedback system required to maintain the constant temperature is provided by having the micro-TCD connected as one of the four components of a Wheatstone bridge.
Rastrello et al. (2012) and Rastrello et al. (2013) disclose a micromachined TCD. Here, two pairs of identical platinum resistors are arranged into a Wheatstone bridge defined over suspended dielectric membranes. One arm of the bridge is designated a reference channel, and the second arm is designated an analytical channel. During use, the analytical channel receives the gas flow output of a gas chromatography column and the reference channel is connected to an empty fused silica capillary or to the carrier gas only.
US 2013/0256825 A1 discloses an integrated circuit which comprises a gas sensor. The sensor described in this document features an electrically resistive sensor element which is located in a position where it is exposed to the sample gas. The circuit also includes a barrier, which forms a trench for the sensor to sit inside, with the intention of inhibiting detrimental effects caused by exposing the sensor directly to the flowing gas.
Romero et al. (2013) discuss a calorimetric method for a combination of flow rate and thermal conductivity measurements. Calorimetric flow sensors usually feature a heater and two temperature sensors, up- and downstream of the heater, with the temperature difference between the sensors indicative of the flow velocity. In this paper, the thermal conductivity is measured by generating heat sinusoidally, at a fixed frequency, allowing the measurements to be made without prior knowledge of the rate of flow. The device can also measure the flow rate of the gas in a DC mode, if the thermal diffusivity of the gas is known.